PHNOM PENH (Reuters) - Cambodian Prime Minister Hun Sen pushed ahead with forming a new government on Tuesday and said he had rejected a demand from the main opposition party, which says it won July's general election, to be given senior positions in parliament.

The opposition Cambodia National Rescue Party (CNRP), which boycotted the opening of parliament on Monday, said it would hold further protests around the country. One man was shot and killed during a rally in the capital, Phnom Penh, last week.

Hun Sen told reporters that, in talks last week, the CNRP had not pressed its demand for an independent inquiry into alleged irregularities during the July poll, but had sought the position of National Assembly president plus six of the 12 committee chairmen's jobs.

"Have you ever seen, anywhere in the world, a minority party holding the position of the president of parliament?" he asked.

The prime minister suggested the real reason for the opposition boycott of parliament was not alleged vote-rigging, but its failure to secure top jobs.

"If we'd agreed to their demand for the presidency, they would surely have attended the meeting," he said.

Hun Sen, 61, has been in power for 28 years and has said he will rule Cambodia into his seventies. He portrays himself as the man who saved Cambodia from the terror and chaos of the Khmer Rouge years in the 1970s and the civil war that followed.

But his authoritarian rule and widespread corruption alienated many young people who did not live through that era and who turned to the newly merged CNRP in the election.

The National Election Committee says Hun Sen's Cambodian People's Party (CPP) won 68 seats in the poll to the CNRP's 55. That was already a big setback for Hun Sen, but the CNRP claimed victory, saying it was cheated out of 2.3 million votes.

Hun Sen has ridden roughshod over critics in the past, but was obliged to hold talks with CNRP leader Sam Rainsy last week as an emboldened opposition took to the streets.

He has taken back the initiative by forcing the opening of parliament in the face of the CNRP boycott, leaving the opposition struggling to keep up the momentum.

"We will hold nationwide demonstrations and we will mount a diplomatic campaign to ensure that the election results and the unconstitutional meeting of parliament are not recognised," CNRP lawmaker Yim Sovann told Reuters.

The Cambodian constitution stipulates that "the National Assembly shall comprise at least 120 members".

The opposition and some political analysts say that means a quorum of 120 lawmakers is needed to open parliament. Monday's session went ahead after the names of the 123 elected members were simply read out and Hun Sen said in a short speech to the house on Tuesday that everything had been done legally.

He says the rules stipulate a new government can be formed if a minimum 63 of the 123 lawmakers vote in favour and his premiership was endorsed by the 68 CPP members on Tuesday.

The names of ministers have not yet been announced, but a first cabinet meeting is due to be held on Wednesday.